It is known to fabricate fiber-reinforced plastic composites (FRP) possessing superior structural strength from strands or tows of reinforcing fibers such as fiber glass enshrouded within a plastic matrix, particularly of cured thermoset resins fabricated under super-atmospheric conditions within a material forming die. Reacting the thermoset reactants and working the FRP within the confines of the die at semi-solid extruding conditions creates compressive forming pressures which yields an extremely compact and dense product. The FRP products are useful for numerous applications where structural strength and durability are required and the more expensive, less durable or scarcer fabricating materials may be effectively replaced with FRP.
These FRP products are typically manufactured by a manufacturing process referred to as pultrusion. The term "pultrusion" is generally used to mean the art for manufacturing fiber reinforced plastics wherein a continuous reinforcing material, most commonly, a filamentary material, is first impregnated with a curable or polymerizable liquid resin and pulled through a die having a desired cross section to shape the impregnated resinous material while polymerizing or curing the reactants into a cured product. The cured product of a continuous length and uniform cross sectional size is pulled from the discharging orifice and cut into pieces of the desired length. The most prevalent curing practice involves simply heating the heat initiated curable reactants to a reaction temperature sufficient to initiate the reactants to undergo chemical reaction and cure into a thermoset or cross-linked resin. Pursuant to this common practice, thermosetting reactants impregnated upon reinforcing strands or tows are continuously fed through a material forming die maintained at a thermally reactive temperature sufficient to cause heated thermosetting reactants to react within the heated die and thereby permit the reactants to cure into a fiber reinforced thermoset resin product.
Pultrusion is particularly useful in the manufacture of FRP composites of a relatively large bulk or dimensional size. The compacting and pressurized processing conditions yield a densely weighted product. Pultrusion is commonly used in the manufacture of bulky fiber reinforced thermoset impregnated materials such as in the fabrication of high-strength materials such as pipes, rods, tubes, levers, handles, construction beams, window and door jams, gates, frames, etc.
It is known that thermosetting resins containing ultraviolet curable reagents may be cured by exposing the resins to ultraviolet light. The ultraviolet light catalyzes ultraviolet sensitive initiating precursors causing the reactants to cure into a thermoset resin. In order for the reactants to uniformly cure into a thermoset resin, the ultraviolet light must necessarily uniformly penetrate the reactants to trigger the catalysis of the reaction system. Although ultraviolet light may be effectively utilized to catalyze thermoset products of a relatively small dimensional size, UV light cannot effectively penetrate into the reaction media such as contained within the forming die as typically used to form bulky materials by pultrusion. Attempts to utilize UV light to cure such bulky materials generally results in topographical or surface curing of the reactants. This provides a defective product possessing a superficial cured thermoset skin leaving the internally disposed reactants uncured. Initiation and curing, in essence, is accordingly limited by the superficial penetration of the UV light within a peripheral margin of thin-skinned layer of reactants under conventional UV curing techniques.
Although there have been attempts to cure UV curable resins with UV light in a pultrusion, the irradiation of the curable resins have been conducted in an open environment in which the UV curable resin is exposed to UV light source. Curing under an open environmental conditions fails to produce the desirable compact and denser product as produced within the closed confines of a material forming die via pultrusion. Attempts have also been made to use transparent dies constructed of materials capable of withstanding the elevated thermal conditions needed to initiate the curing reaction. Material forming dies constructed of glass have been proposed for this purpose. Unfortunately, glass dies only allow the interfacing superficial margin of UV curable resin to become initiated and cured by UV light leaving the internal portions uncured. Glass dies also do not possess sufficient strength and durability as provided by metal dies in pultrusion. Moreover, uncured resins characteristically possess a high degree of tack with a tendency to stick onto the inner surface of the material forming die which, in turn, leads to fouling and plugging of the die. This results in destruction of the die forming efficacy as well as the production of an uncured and defective FRP.
Curable resins which, under actinic radiation conditions, cure into a solid cured reaction product have been extensively reported and commercially available from a variety of commercial sources. The following literature which discloses various different actinic ray curable resinous reactants which, upon actinic radiation, will cure into a thermoset resin are incorporated into and made a part of the enabling embodiment of this application. Exemplary of such actinic curable resins include the acrylic and methacrylic acids, amides and esters as disclosed in U.S. Pat. No. 4,892,764 (e.g. see Col. 6, line 53 through Col. 7, line 15) and U.S. Pat. Nos. 4,051,195; 2,895,950; 3,218,305; and 3,425,988; the actinic curable vinyl monomers such as styrene vinyl toluene, vinyl pyrrolidone, vinyl acetate, divinyl benzene and the like; the actinic radiation curable unsaturated polyesters solubilized in vinyl monomers, as ordinarily prepared from alpha-beta ethylenically unsaturated polycarboxylic acids and polyhydric alcohols such as described for example in U.S. Pat. No. 4,025,407; the UV curable epoxy resins, including the cycloaliphatic epoxides, the diglycidyl ether of resorcinal (WC69), the diglycidyl ether of bisphonel-A (EP828), the diglycidyl ether of cyclohexane dimethonal (MK107) the limonene dioxide, the limonene oxide, alpha pinene oxide, aliphatic epoxides such as butyl diglycidyl ether, the diglycidyl ether of 1,4 butanediol (WC67), neopentyl glycol diglycidyl ether, diglycidyl ether of neopentyl glycol (WC68) as illustratively disclosed in U.S. Pat. No. 4,412,048.
"Actinic radiation" is generic to ultraviolet light and generally refers to electromagnetic radiation having a wavelength of about 700 nm or less which is capable, directly or indirectly, of curing the specified resin component or the resin composition. Photoinitiators conventionally added to actinic radiation curable resins in an amount effective to respond to the actinic radiation and to initiate or to induce curing of the curable resin may be incorporated at initiating levels into the curable resins. Representative known photoinitiators useful with ultraviolet (UV) actinic radiation in the curing of (meth)acrylic and vinyl monomers include free radical generating UV initiators such as benzophenone, diethoxy-acetophenone, benzoin methyl ether, benzoin ethyl ether, benxion isopropyl ether, diethoxyxanthone, chloro-thio-xanthone, azo-bis-isobutyronitrile, N-methyl diethanol-amine-benzophenone and mixtures thereof. Known visible light initiators include camphoroquinone-peroxyester initiators and 9-fluorene carboxylic acid peroxyesters. Art-recognized infrared initiators include cumeme hydroperoxide, benzoyl peroxide, asobisisobutyronitrile, and like azo and peroxide compounds.
When actinic radiation is applied to an epoxy resin, photoinitiators which, when exposed to UV light, liberate a Lewis acid and/or Bronsted acid, such as iodonium salts, sulfonium salts, arsonium salts and diazonium salts may be used to initiate the epoxide reaction. The required photoinitiator level for initiating any given reaction is known and may be readily determined by conventional techniques. In general, when (meth)acrylic and vinyl resin components are used, the photoinitiator concentration will typically range from about 0.1-10 percent by weight of the total weight of the resin component. For the UV curable epoxies, the Lewis acid/Bronsted acid-releasing initiator will typically be present at a concentration of about 1-5 percent by weight, based on the weight of the epoxy resin.
Auto-curable multi-acrytate resin which, when exposed to ultraviolet irradiation, will cure into a thermoset resin without requiring conventional photoinitiators are also known. An article by Wen-Yen Chaing et al. entitled "UV-Autocurable Epoxy-Multiacrytate Resins" (Journal of Applied Polymer Science, Vol. 43, Pages 1827-1836, 1991) reports the syntheses of various UV autocurable epoxy-multiacrylate resins by reacting various diglycidyl ethers of 1,4-butanediol, neopentyl glycol, resorcinal, cyclohexane dimethanol and bisphenol-A with acrylic oligomers and monomers such as 1,2-hydroxyethyl acrytate and 3,3', 4,4'-benzophenone tetracarboxylic, dianhydride to provide built-in photoinitiating systems. U.S. Pat. No. 4,004,998 by G. Rusen, U.S. Pat. No. 4,514,527 by R. L. Bowen, U.S. Pat. No. 4,158,618 by S. D. Pastor and polyurethane and polyesters multiacrylate oligomers (e.g. see Chiang et al., Journal of Applied Science, Vol. 37, Page 1669, 1989 and Vol. 41, Page 2971, 1990; Angew Makromol, Chemistry, Vol. 179, Page 57, 1990) also disclose UV curable resins which cure without requiring added photoinitiator. The autocurable resins were developed to overcome the need to load the resin with large amounts of photoinitiators which often failed to homogeneously mix and remain chemically unbounded within the cured resin.
In the pultrusion process, the curable resin comments are typically formulated with volatile constituents which, when exposed to the elevated reaction conditions, will volatilize. Such volatilized constituents can form pin holes, pockets, channels, voids, etc. throughout the product which renders the product less appealing as well lead to defects or substantial weakening to the FRP structure. The range of useful reactants and additives can also be limited by the compatibility of the particular reactants and additives to the relatively high temperatures and heat needed to drive the chemical reaction to completion or cure the thermoset resin. The elevated thermal reaction conditions can also give rise to fluid movement of certain components within the uncured resin which, in turn, may adversely affect the uniformity of the cured product. In addition to the cured reaction by-products, other conventional additives such as coloring additives (e.g. dyes, pigments), stabilizers, antioxidants, fillers, surfactants, tackifiers, promoters, solvent carriers, and dispensers, etc. are limited in adaptability and can create residual problems due to harshness of the thermal reaction conditions created by the heated die.
It would be particularly advantageous to alleviate the problems associated with the use of heated conditions to cure bulky materials formed by the pultrusion technique by using actinic radiation such as ultraviolet light to catalyze the reactants within a material forming die to a cured thermoset product. The ability to create FRP pultrusion products under ambient die conditions would substantially alleviate those problems associated through the use of heated dies in the pultrusion of FRP products. Significant benefits would arise if it were possible to uniformly initiate curing of FRP composites with a material forming die using ultraviolet light to initiate curing under processing conditions so as to impart substantially uniform curing to the FRP.